This invention relates to an advancing device for close-up lens.
In general, a close-up lens requires a large amount of advancing and therefore a provision of a precision advancing mechanism therein. Accordingly, the entire length of the close-up lens may be long even in the state before advancement and a distance between a tip end of the lens cylinder and a first lens may also be long. Therefore, there are some disadvantages in close-up photography. For example, if an object to be photographed is a living thing such as an insect, when the lens approaches thereto, it may move, thereby missing the opportunity for a picture. Also, when the object is placed in a hole or a groove, focusing may be impossible and at this time, flash-photography may be necessary. However, the light emitted may not reach the object due to the width of the hole. Furthermore, when photographing parts of the human body during medical procedures, forceps must be placed around the body which is the object of the picture and when photographing teeth, the lens must be inserted into the mouth. Accordingly, it is desirable to make the distance between the object and the tip end of the lens cylinder (called "operation distance" hereinafter) as long as possible. Therefore, in designing a suitable close-up lens it is necessary to bring the principle point close to the object, so that the lens can be positioned as far from the object as possible.
As mentioned, because of the necessity of the advancing mechanism in the close-up lens, the length of the lens cylinder is long even in the state before advancement and also, the distance between the tip end of the lens cylinder and the first lens becomes longer. This may cause an eclipse of incident light due to the tip end of the lens cylinder and also, even through an attempt to prevent such eclipse is made, it results in an increase of the diameter of the lens. Moreover, since the lens is placed deeply within the lens cylinder, it is difficult to attach a filter, etc. to the lens.